Our studies have shown that reticulocyte cell membrane receptors preferentially react with diferric transferrin from which they abstract a single unique ferric ion (from site A). The resultant monoferric transferrin bearing its ferric ion at site B is then displaced from the receptor site by an incoming diferric transferrin molecule or by an incoming monoferric molecule bearing iron at the B site. In the latter case, this monoferric bearing its B site iron is now oriented so that scission of its iron occurs and the resultant apoferric molecule is displaced by an incoming molecule (either a diferric or a monoferric B site transferrin). Monoferric transferrin bearing iron at the A site does not surrender its iron to the cell. These findings suggest that this monoferric A site bound transferrin might serve as the iron donor to other cells where the metal is essential. Studies will be performed using I131 tracer labelled protein to determine whether this species reacts with the receptor but does not yield its iron or whether it is unreactive.